The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the UMTS (Universal Mobile Telecommunication Service) system, and LTE (Long term Evolution) is now under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach data rates of more than 100 Mbps on the downlink and of more than 50 Mbps on the uplink. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN).
Generally, one or more cells are allocated to a radio base station, known in the 3GPP LTE system as eNB (enhanced/evolved NodeB) or eNodeB. In addition, eNBs in LTE will interact directly with the core network and with other eNBs. A plurality of user equipments can be placed in a cell served by an eNB. A user equipment (UE) can be represented by a mobile phone, a wireless terminal, a laptop, a personal computer, a personal digital assistant, a voice over interne protocol (VoIP) capable phone or any other 3GPP LTE capable UE. Generally, a UE's first access to the system is performed by means of a random access (RA) procedure. The objectives of the RA procedure may include: initial access; handover; scheduling request (request for radio resources); timing synchronization, and the like. The radio network nodes generally control the behavior of the UE. As an example, uplink transmission parameters like frequency, timing and power are regulated via downlink control signalling from the radio base station (e.g. eNB) to the UE. For the uplink (UL) frequency and power estimate parameters, a UE can derive those parameters from one or several downlink (control) signals. However, making a timing estimate for the uplink is more difficult due to that the propagation delay between the eNB (or eNodeB) and the UE is generally unknown. As an example, when a UE is powered on or turned on or after a long standby time, the UE is not synchronized in the uplink. Therefore, before commencing traffic, the UE has to access the network, which in a first step includes obtaining synchronization to the network. This is usually done by the UE which performs measurement(s) by listening to downlink signals and obtain from these signals timing synchronization; an estimate of a frequency error, and also an estimate of the downlink pathloss. Even though the UE is now time-synchronized to the downlink, signals to be sent from the UE are still not aligned with the reception timing at the eNB (or eNodeB) due to said unknown propagation delay. Thus the UE has to carry out a random access (RA) procedure to the network. The RA procedure is a procedure typically used by the UE to request access to a system or resources when the UE discovers a need to acquire uplink synchronization or a need to make an uplink transmission and no resources for said uplink transmissions are yet available to the UE. Furthermore, synchronization or time alignment of uplink transmissions aims to minimize interference with transmissions of other UEs and increase resource efficiency by minimizing the need for guard bands.
The RA procedure can be classified into a contention-based random access procedure and a contention-free (or non-contention-based) random access procedure.
For the contention-based random access procedure, a first set forming a pool of non-dedicated random access preambles is assigned per cell (i.e. to a eNodeB). This pool is primarily used when there is UE-originated data and the UE has to establish a connection and an adequate uplink timing relation with the network through the RA procedure. When performing contention-based random access, the UE arbitrarily selects a preamble from the pool as the non-dedicated random access preamble. This is known as UE initiated random access (supported in LTE). Thus for contention-based random access, the network (or the eNB) is not (immediately) aware of which UE selected which preamble. A drawback with this is that multiple UEs may in fact select the same preamble and they may attempt to access the network (or eNodeB) at the same time. This may cause collision(s) to occur. Thus, an extra step of identifying UEs trying to access the network (or eNodeB) and resolving potential collisions, a so-called contention resolution mechanism, is needed.
For performing contention-free random access, there is also defined a second set forming a pool of random access preambles assigned per cell (i.e. to a eNodeB). These preambles are known as dedicated random access preambles. Contrary to the non-dedicated random access preambles, a dedicated random access preamble is assigned to the UE by the eNodeB. In other words, this preamble cannot be autonomously selected by the UE and therefore, for the duration of the validity of the assignment, this dedicated random access preamble is exclusively dedicated to the UE. This is known as network triggered or network ordered random access (supported in LTE). Since a specific preamble is assigned/dedicated to the UE, it is a benefit of contention-free access that the eNodeB can immediately know from the received preamble, which UE tries (or tried) to access the network. This thus eliminates the need for contention resolution and therefore improves resource efficiency by minimizing the risk of collisions. Furthermore, avoiding the contention resolution procedure reduces the delay.
It should be noted that the network-triggered (e.g. in the E-UTRAN) random access can be used to force a UE, which does not have a valid uplink timing to synchronize its uplink to the timing at the eNodeB, e.g., prior to the eNodeB making a downlink transmission for which the UE will need to transmit an acknowledgment (ACK) or a negative-acknowledgment (or a non-acknowledgment) (NACK) feedback. It should also be mentioned that because of the non-zero duration of the random access and uplink synchronization procedure, the resynchronization is typically forced in advance of making the downlink transmission. In general a UE (e.g. a mobile terminal) looses synchronization if it is not active for a certain time. It is thus possible to define a decision criterion in the network (e.g. in the E-UTRAN) that after a certain time without any activity a UE (e.g. a mobile terminal) is assumed to have lost synchronization to the eNB.
If e.g. downlink data arrives at the UE, synchronization needs to be re-established first, and for this purpose, the above described dedicated random access preamble is assigned to the UE, which the UE can use to perform a contention-free random access procedure. This will trigger a timing advance adjustment command from the eNB, and based on this command, the UE can re-establish time-alignment. For instance, in LTE (or E-UTRAN) the physical downlink control channel PDCCH is used to assign a dedicated preamble.
Since the random access procedure is the first procedure performed by the UE to access the network, it is important that random access works as it should. If random access fails, the UE cannot access the network. An exemplary scenario where a random access procedure can fail or cannot be performed is when all of the dedicated preambles are already in use. As mentioned before, the eNodeB keeps track of the dedicated preambles it has already allocated/assigned, and when there is no such dedicated preamble available for allocation, the eNB has to advice/refer other UEs, for which there is no dedicated random preamble, to perform a contention-based random access. However, as mentioned before, contention-based random access leads to potential collisions and additional delays when a contention resolution needs to be performed. If collisions occur and one or more UEs fail with their random access, one or several UEs may retry the random access procedure after a certain time which leads to that a back-off mechanism may be triggered that introduces further delays. A back-off mechanism is an approach to control UE re-accesses attempts by introducing e.g. a back-off parameter that represents a period of time to wait before an access reattempt. Thus, the larger the number of re-access attempts, the longer the delay.